Thursday, July 1, 2010

Macro Lenses, Close Up Filters, and Extension Tubes

I happen to have some inexpensive extension tubes that I bought from China on eBay one day a few years ago. I also have a couple of Canon 500D achromat (multi-element corrected) close up filters in 58mm and 77mm. And I have a nice Sigma 105mm 2.8 Macro lens that can focus on its own to 1:1 magnification.

So what does 1:1 magnification mean? Well, it’s pretty simple … the sensor on my D700 is 36mm across (same as 35mm film frame) and if I focus at closest distance with the Sigma, I get a 36mm cross section of the subject on the sensor. This is also known as life-sized.

Now … when you shoot macro, you are often trying to get higher magnifications than that … 2:1 and even 3:1 (18mm cross section and 12mm cross section respectively) can get you shots of the individual elements of a bug’s eyes. For example, here is a dragonfly image shot with my D70s and the Sigma 105mm 2.8 macro with Tamron 1.4x teleconverter and Canon 500D achromat. I lit it with flash, which kills camera movement and allows for small apertures for depth of field. The setup looks like this:


Magnification of this setup was measured at 2.1:1 … a bit larger than twice life-sized.

Here is the dragonfly and the crop … you can see that I got quite a bit of detail here …



So … back to this experiment. I thought I’d try a full blown setup with extension tubes against 2x Kenko teleconverter and Sigma 105 against my new Nikon 105 AI … the Sigma shot with and without the Canon 500D achromat. I did not do all combinations, but I did enough of them to get interesting …

A note on how I focused – I set each lens at closest focus and then used a macro rail to focus the camera. I then stopped down to shoot (these extension tubes are manual.) The rail looks like this:


This is as precise a focus mechanism as you can use. And if you want pin point focus, you can enable live view and focus a magnified image.

The first setup is the D700 with the extension tubes (full set) and the Sigma 105 and 500D.


A close up of the 500D.


This produces the image:


This captures a cross section approximately 15.8mm wide. That makes it 36/15.8 = 2.28:1 magnification … not too shabby.

How about with the 500D?



That’s 18.1mm for about 1.98:1 magnification. Not bad at all …

Now let’s try the Nikon with the tubes but no 500D (I have step up rings and might just use them sometime with this setup, but not today) …



That’s a 28mm cross-section … for 1.3:1 magnification … wow … that’s pretty good for an older lens that focuses at a long distance. Hmmm ….

Switching to the teleconverter and put the 500D back on … this introduces extra glass in the system, so should degenerate the image somewhat. But we’re really only looking at magnification right now …



Wow … I count only 15.5mm in that cross-section … so we get 2.32:1 magnification, very slightly edging out the extension tubes. And the viewfinder remains much brighter …

With the TC and the Nikon (no image of this setup), the magnification is disappointing.


So is the dust, but let’s pretend we didn’t see that :-)

137mm … that’s 1:3.8 magnification, or about 1/4 life-sized … this is perfect for flowers by the way. So a useful combination may have been born. Gotta try it on flowers. Stay tuned.

I find the tubes rather inconvenient as the light drops dramatically in the viewfinder. The 2x TC is easy to use and works really well with these lenses. Magnification is high and sharpness seems ok. Add the 500D and you really have something if you are bug hunting …

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